Radiotherapy treatment is generally distributed to a patient in a number of sessions, or fractions. Before the treatment starts, a planning image of the patient is obtained. The planning image provides input data to a treatment plan, which defines the treatment to be given to the patient. Before each session a fraction image is obtained, to assist in positioning the patient with respect to the treatment unit before the delivery. The fraction images can also be used to assess the dose actually delivered to the patient during the session and also to study changes in patient-geometry that have occurred since the planning image was acquired. Such changes are important for treatment evaluation and the result of the evaluation may lead to a decision that modifications of the treatment plan are required. In the context of this description, both the planning image and the fraction images are 3D images constructed from a number of 2D images constituting projections of the patient's body.
Dose planning requires information about the location of the various organs and also about their material properties, such as density and/or atomic composition. Density information is used for dose planning. If photon radiotherapy is used, the density and atomic composition determine the attenuation of the radiation. If ion radiotherapy such as proton radiotherapy is used, the density and atomic composition determine the stopping power, which affects the distance that the ions will travel within the patient's body. For the initial planning, this information is taken from the planning image. The fraction images are typically used to determine the new boundaries of the regions of interest in order to aim the radiation beams correctly.
Therefore, the planning image should comprise information not only about the contours but also about the material properties of each region of interest. As the geometry of the tumor and other tissues changes during the course of therapy, the fraction images are used to obtain up-to-date contour information. However, the fraction images may have considerably less information than the planning images. For example, a fan beam CT scan (referred to in this document as CT) may be used for the planning image while Cone Beam CT (CBCT) scans are used for the fraction images. CT images comprise all the information needed for dose planning but are relatively expensive and involve a higher radiation dose to the patient than CBCT. CBCT on the other hand, with the advantage of giving lower radiation dose to the patient and also being a less expensive imaging system, does not always provide reliable information about material properties and in particular is subject to distortion such as cupping distortion, where the intensity of the image is misrepresented near the edges of the image. Other imaging technologies involve even less or no radiation but do not provide all the information necessary for proper treatment planning.
Typically, the field-of-view for a CBCT image does not cover the full patient outline. This means that when computing dose based on a fraction image the densities for the parts outside of the field-of-view need to be estimated for dose computation. One solution would be to superimpose the patient's outline from the planning image onto the fraction image and assuming that it has the same properties as water. Water is a reasonable compromise, as it is a good approximation for most parts of the body, but this solution still causes inaccuracies in the dose computation.
Ruchala, Olivera, Kapatoes, Reckwerdt and Mackie: Methods for improving limited field-of-view radiotherapy reconstruction using imperfect a priori images, Medical Physics 29, 2590 (2002); doi: 10.1118/1.1513163, discloses a method of handling this problem working directly in the 2D images which are afterwards used to construct the 3D image. The method proposed in this document is not applicable when working directly with 3D images. Typically, the treatment planning system does not provide access to the 2D images so any changes made through these systems must be made in the 3D image.